1. Field of the Invention
The present invention concerns mobile telephones and in particular a self-disabling device for disabling a portable terminal that has been stolen or used in an unauthorized manner.
2. Description of the Prior Art
In a GSM type mobile telephone system each user has a portable terminal or telephone which he can use to telephone any other telephone user. Each portable terminal is identified by a mobile equipment identifier number (IMEI) known to the network control station.
Each network user is supplied with a subscriber identifier module (SIM) when he takes out a subscription. He can use his portable terminal only after inserting his SIM card into his terminal. However, the SIM card is totally independent of the terminal and can be used in different terminals.
In the case of a terminal that has been stolen, the thief or someone receiving the stolen terminal can take out a subscription and obtain a SIM card and in this way obtain access to the network. For this reason the control station maintains an up-to-date database listing stolen terminals. To prevent a stolen terminal using the network, the IMEI of the terminal is requested during setting up of the session with the terminal, this identifier is checked against the database and communication with the terminal is barred if it proves to have been stolen. A procedure of this kind is time-consuming and greedy in terms of resources in that the verification procedure uses the communication channel of each terminal although very few terminals are stolen.
For this reason the main object of the invention is to provide a self-disabling device for disabling a portable terminal that is simple and effective and which does not make major use of resources assigned to the terminal.
Another object of the invention is to provide a self-disabling device for disabling a portable terminal which has been stolen in response to information contained in a short information message transmitted systematically to all terminals.